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By David Robson If you think the idea of gravitational waves propelling interplanetary spacecraft sounds like science fiction, you’re in good company – any astrophysicist will rubbish the idea out of hand. However, that didn’t stop the US Defense Intelligence Agency (DIA) from commissioning a report to investigate whether the elusive waves could pose a threat to US security. The JASON Defense Advisory Group were also asked to judge whether high-frequency gravitational waves could image the centre of the Earth, or be used for telecommunications. Gravitational waves are ripples in space-time caused by the movement of an extremely large mass, such as a very dense star. Yet even those from huge stellar events have been too weak to trip the most sensitive detectors. The best evidence is indirect, coming from observations of how superdense, binary neutron stars lose energy. Nevertheless, the JASON team was asked to consider a funding proposal from US company GravWave to the DIA that claimed humans could generate strong gravitational waves on Earth, using the Gertsenshtein effect. This describes how electromagnetic waves travelling through a very strong magnetic field can be converted into gravitational waves. When the JASON team did the maths, however, results were not good for the plan’s supporters. The technique is so inefficient that it would take longer than the lifetime of the universe for every power station on Earth to produce a gravitational wave with the energy of one ten millionth of a Joule. Accelerating a spacecraft at 10 metres per second squared, a rate that just exceeds the pull of Earth’s gravity, would require 1025 times (a 1 followed by 25 zeroes) the electricity output of the world. The report (pdf format) concludes: “These proposals belong to the realm of pseudo-science, not science.” Physicists striving to actually detect gravitational waves expressed surprise that a committee needed a 40-page report to come to that conclusion. “The proposal is utter nonsense,” says Karsten Danzmann from the Max Planck Institute for Gravitational Physics in Hanover, Germany, and member of the GEO600 project to detect gravitational waves. “I’m a bit surprised the agency bothered to commission an investigation – it would probably have been enough to just ask an in-house science advisor,” he says. David Shoemaker, from MIT in Cambridge, Massachusetts, a member of the LIGO project to detect gravitational waves, agrees that a quick phone call to a physicist may have been sufficient. But he quips that given the US defence establishment’s history of funding bad science, over-long reports that rubbish such ideas at an early stage may not be a bad thing. “The Department of Defense always have a few projects on the go that disobey the rules of thermodynamics, so I wish they would commission this kind of in-depth study in more cases.” In the mid-1990s and early 2000s the Pentagon spent millions of dollars on developing a quasi-nuclear weapon called the hafnium bomb that was actually based on junk science. When put into that context, perhaps the money spent on a report that prevents similar spending on gravitational wave weapons was actually a good investment. More on these topics: